Solar cells have been attracting attention as an environment-friendly electric energy source. Now, inorganic substances, such as monocrystalline silicon, polycrystalline silicon, amorphous silicon and compound semiconductor, are used as a semiconductor material of a photovoltaic device of a solar cell. However, solar cells produced using inorganic semiconductors have not spread widely for home use because of their cost higher than that of power generation systems such as thermal power generation and nuclear power generation. Such a high cost is derived mainly from the process of producing a semiconductor film in vacuum at high temperatures. Therefore, to simplify the production process, organic solar cells using organic semiconductors such as conjugated polymers and organic crystals or organic dyes as semiconductor materials have been investigated.
However, the most serious problem with organic solar cells using organic semiconductors such as conjugated polymers and the like is that such solar cells are low in photoelectric conversion efficiency in comparison with solar cells using conventional inorganic semiconductors, and therefore such solar cells have not been used practically, yet. The following three points are major reasons for the low photoelectric conversion efficiency of organic solar cells using conventional conjugated polymers. The first reason is that solar light absorption efficiency is low. The second reason is that a bound state called exciton in which an electron and a hole generated by solar light are resistant to separation is formed. The third reason is that since a trap which captures a carrier (electron or hole) is likely to be formed, a produced carrier tends to be captured by the trap, resulting in low mobility of carriers. In other words, while a semiconductor material is generally required that the carriers of the material has a high mobility μ, there is a problem that conjugated polymers have mobilities μ being lower than those of conventional inorganic crystalline semiconductors or amorphous silicon.
Therefore, finding the means for successfully separating a produced electron and a produced hole from exciton and the means for preventing carriers from scattering in an amorphous region of a conjugated polymer or between conjugated polymer chains or from being captured by the trap to increase the mobility is the key for bringing solar cells using organic semiconductors in practical use.
The hitherto known photoelectric conversion devices using organic semiconductors can now be generally classified into the following elemental constitutions; that is, a Schottky type in which an electron donating organic material (p-type organic semiconductor) and metal with a small work function are joined, and a heterojunction type in which an electron accepting organic material (n-type organic semiconductor) and an electron donating organic material (p-type organic semiconductor) are joined, and so on. In such devices, since only organic layers (almost several molecular layers) in a junction contribute to the generation of a photocurrent, the photoelectric conversion efficiency is low and, and therefore the improvement in the efficiency is a pending problem.
One approach for increasing the photoelectric conversion efficiency is a bulk heterojunction type in which an electron accepting organic material (n-type organic semiconductor) and an electron donating organic material (p-type organic semiconductor) are mixed so that joined surfaces which contribute to photoelectric conversion are increased (for example, J. J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Marseglia, R H. Friend, S. C. Moratti, A. B. Homes, “Nature” No. 376, p. 498, 1995). In particular, photoelectric conversion materials using a conjugated polymer as an electron donating organic material (p-type organic semiconductor) and using fullerene such as C60 or carbon nanotubes as well as a conducting polymer having n-type semiconductor properties as an electron accepting organic material have been reported (for example, E. Kymakis, G. A. J. Amaratunga, “Applied Physics Letters” (U.S.A.), Vol. 80, p. 112, 2002, G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, “Science”, Vol. 270, p. 1789, 1995, Japanese Unexamined Patent Application Publication No. 2003-347565 and Japanese Unexamined Patent Application Publication No. 2004-165474).
Moreover, a photoelectric conversion material has been reported which comprises an organic semiconductor with a band gap having been reduced by the introduction of an electron donating group and an electron attracting group into a main chain to cause the organic semiconductor to efficiently absorb the radiant energy of a wide range of the solar light spectrum (for example, E. Bundgaard, F. C. Krebs, “Solar Energy Materials & Solar Cells”, Vol. 91, p. 954, 2007). Strenuous researches are made to thiophene skeletons as the electron donating group and to benzothiadiazole skeletons as the electronic attracting group (for example, X. Li, W. Zeng, Y. Zhang, Q. Hou, W. Yang, Y. Cao, “European Polymer Journal”, Vol. 41, p. 2923, 2005, E. Bundgaard, F. C. Krebs, “Macromolecules”, Vol. 39, p. 2823, 2006, U.S. Unexamined Patent Application Publication No. 2006-52612, Japanese Unexamined Patent Application Publication No. 2003-104976, U.S. Unexamined Patent Application Publication No. 2006-174937 and U.S. Unexamined Patent Application Publication No. 2004-115473). However, sufficient photoelectric conversion efficiency has not been obtained, yet.
As described above, all of such conventional organic solar cells have a problem that photoelectric conversion efficiency is low. It could therefore be helpful to provide a photovoltaic device with high photoelectric conversion efficiency.